Potato growers are at risk of virus and disease threat if they do not follow an integrated approach to new desiccation regimes, say experts working on the third year of desiccation trials taking place across AHDB Farm Excellence sites.

Results and observations at AHDB trial sites have shown that the slower ‘kill’ achieved by the chemical and mechanical alternatives to diquat means that green stems and leaves can still be present up to three weeks after desiccation sprays. Even very small amounts of ‘green material’ remain a viable target for virus-carrying aphids and diseases, such as blight and gangrene.

In a recent AHDB webinar, seed growers were asked when they would apply their last insecticide to the crop – with 59 per cent responding it would be over two weeks before a T1 desiccation spray.

Eric Anderson, Senior Agronomist for Scottish Agronomy, and leading the trials at Milton of Mathers Farm near Montrose, the AHDB Strategic Potato (SPot) Farm in Scotland, said “Not all growers will be thinking about virus threat when desiccating, but you can find aphids on green regrowth up to three weeks after your T1 spray. Systemic insecticides generally remain effective for 14 – 21 days, with pyrethroids only lasting seven days – so if your last insecticide was applied two weeks before desiccation, you’ve got a problem.

“Even for ware and processing growers, the presence of green leaves and stems means the threat of foliar and tuber blight, which needs to be managed.”

Practical tips for growers

The main challenge for ware growers is achieving adequate skinset, and research to date has shown this is possible without diquat, though there are some factors to take into account:

• Skinset is slower in wet soils –  stop irrigation seven days before desiccation
• There will be between a one and four-day delay to skinset with chemical desiccation, so adjust your schedule
• Aim for an early-mid morning application of desiccants, ideally on a sunny day.

Dr Mark Stalham, Head of NIAB CUF is leading AHDB trials south of the border across AHDB SPot Farms.  He said: “We found big differences in the rate that different options killed off the canopy, but interestingly far fewer differences in skinset than you might have expected by looking at the canopies. Rapid canopy death does not always mean rapid skinset.”

Eric Anderson added: “The key to good desiccation is attention to detail. It’s all about application, application, application. The product is only half of your end result, the other half will come from the timing, nozzle choice, water volume and the skill of the operator.”

AHDB field-based research and demonstrations on desiccation without diquat started in 2018 and has taken place on eight trial sites across Britain, giving growers a chance to see alternative methods on real crops and discuss results with researchers, agronomists and their peers.

This year an August webinar will be followed by another digital meeting in September, and growers can review all results and observations at: https://ahdb.org.uk/knowledge-library/desiccation-of-potato-crops